This book explains the geological forces behind the Cascadia subduction zone and the evidence — tree-ring-dated “ghost forests,” coastal sediments, Indigenous oral histories and far-off tsunami records — that reveal the region’s propensity for rare but catastrophic megathrust earthquakes and tsunamis roughly every several hundred years. It walks readers through how scientists reconstruct past events, model worst-case scenarios for a magnitude ~9 temblor, and trace the likely chain of impacts on modern society: collapsed buildings and highways, ruptured water and sewer lines, prolonged loss of power and communications, widespread tsunami inundation and cascading economic and humanitarian crises. Interwoven with human stories and policy analysis, the book emphasizes preparedness, risk communication and the tough decisions that communities and governments must make to reduce future loss of life and infrastructure.